1. Field of the Invention
The present invention relates to a planar optical waveguide type optical module and a method of manufacturing the same, more particularly, to an optical waveguide platform capable of mounting an optical device by a flip-chip bonding and a method of manufacturing the same.
2. Description of the Prior Art
Recent technologies require the development of highly functional opto-electronic components. Opto-electronic hybrid integration using a silica planar lightwave circuit (PLC) is one promising method to satisfy the requirement. The combination of the PLC and active device leads to ahighly functional opto-electronic module for signal processing.
In manufacturing optical device on silicon substrate using a flame hydrolysis deposition and PE-CVD (plasma enhanced chemical vapor deposition), optical passive device such as optical waveguide can be fabricated, but active device such as a laser diode (LD) or a photo diode (PD) can not be integrated with ease.
A hybrid integration platform is an integrated opto-electronic circuit board, which includes an optical active device mounting plane and optical waveguide and electrical wiring. The hybrid integration platform can satisfy both the optical passive device function and the optical active function, for highly functional signal processing such as optical switching, optical signal transmission and so on.
In this case, the optical connection loss must be sufficiently small by mounting and aligning a small-sized optical active device precisely with the optical waveguide. Accordingly, an optical waveguide platform capable of mounting the optical active device using flip-chip bonding was reported.
Hereinafter, a first conventional method of manufacturing the optical waveguide platform will be explained with reference to FIGS. 1A to 1D.
Referring to FIG. 1A, a given area of a silicon substrate 101 is anisotropically etched to form a terrace 102. A lower clad layer (silica layer) 103 is formed on the terrace 102. The lower clad layer 103 has unevenness in the surface due to the step of the terrace 102.
Referring to FIG. 1B, in order to remove the surface unevenness of the lower clad layer 103 due to the step of the terrace 102, the lower clad layer 103 is polished and it's surface is planarized. A core layer is formed on the lower clad layer 103 and patterned to form core waveguide 104 and then an upper clad layer 105 is formed to cover the planarized lower clad layer 103 and the core waveguide 104.
Referring to FIGS. 1C and 1D, a predetermined area of the upper clad layer 105, the core waveguide 104 and the flattened lower clad layer 103 are dry-etched so that the terrace 102 is exposed to mount an optical device therein. Numeral number 106 represent a trench in which the optical device is mounted. An insulating film 107 is formed in the trench 106 and metal layers 108 are formed on the insulating film 107 in the trench 106 to make an electrode and an UBM (Under Bump Metal) pad.
Next, solder is deposited on the UBM pad which is composed of the metal film 108 and an optical device 109 is mounted on it. The optical device 109 is generally a semiconductor chip such as a LD or a PD. A metal wire 110 is interconnected electrically between the backside of the optical device 109 and the metal electrode 108.
In the above-mentioned conventional method of manufacturing the optical waveguide platform, a silicon etching process is required in order to make the terrace. Because of the unevenness generated in the lower clad layer due to the step of the terrace, the polishing process is required to planarize the lower clad layer. In the process of precisely etching the silicon to form the terrace, a separate photolithography process using a mask is needed. Furthermore, a terrace pattern can not be manufactured accurately if crystallized direction of the silicon wafer is wrong. Also, in polishing the lower clad layer of the optical waveguide, since the silica film of the lower clad layer has a thickness of several tens μm, it is difficult to polish the lower clad layer precisely. And, if the deviation in the polished thickness exists, the lower clad layer can be partly lost so that it affects the characteristics of the optical waveguide. In particular, in case of a general silica optical waveguide, the substrate could be bent. The bending of the substrate is caused by the difference between the thermal expansive coefficients of the silica film and the silicon substrate. Because of this bending of substrate, a precise polishing without the deviation of the thickness can not be accomplished. Accordingly, there is a problem that a separate process such as a heating treatment is needed for preventing the substrate from being bent.
Hereinafter, a second conventional method of manufacturing the optical waveguide platform will be explained with reference to FIGS. 2A to 2D.
Referring to FIG. 2A, a lower clad layer 202 and a core layer 203 are sequentially formed on a silicon substrate 201 and the core layer is patterned to form core waveguide. A trench area is dry-etched to a needed depth in which optical devices can be mounted.
Referring to FIG. 2B, etch stopper 204 is deposited and patterned on the etched area. An upper clad layer 205 is formed on the lower clad layer 202, the core layer 203 and the etch stopper 204.
Referring to FIG. 2C, the upper clad layer 205 in the trench area is etched so that etch stopper 204 is exposed. A metal and solder 206, in order to mount the optical active device and supply the power to drive the device, is on a given area of the etch stopper 204 in the trench area.
Referring to FIG. 2D, an optical device 207 is mounted on the metal and solder 206 with a flip-chip bonding method.
In the above-mentioned conventional method, after the core layer is etched, the etch stopper must be formed. During the processes, the fine optical waveguide pattern having a thickness of several μm is apt to be damaged by the mechanical impact such as a mask contact. Furthermore, in a subsequent high-temperature process of forming the upper clad layer, since the etch stopper is basically composed of a material different from the silica, it is difficult to form an accurate terrace pattern due to the pattern variation or the crack of the etch stopper. The characteristics of the optical device can be affected by the possible deformation of the core layer and the change of the refractive index, which are caused by the oxidation or the corrosion of the etch stopper.